[Issue 22244] Key tail immutability should allow hashmap to be impl converted from immutable to mutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22244

--- Comment #1 from FeepingCreature <default_357-line at yahoo.de> ---
Simpler demonstration of the issue:

```
immutable int[string] foo;
immutable(int)[string] bar = foo;
```

This should work, because if we cast it:

```
immutable int[string] foo;
immutable(int)[string] bar = foo;
bar["foo"] = 5; // cannot modify `immutable` expression `bar["foo"]`
```

we see that the original `foo` is still protected. So the implconv should go
through.

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